Age of Animals

Ordovician Period, 485.4 - 443.4 MYA

Shallow seas covered large area of the world, lichen-like plants had begun to adapt themselves to life on land. Post-Cambrian trilobites had diversified into many forms including the Cryptolithus shown in bottom of Figure 02a. Echinoderms were characteristic inhabitants in this period. To the left is a primitive species that carried its bulging body on a short stalk. Another species (right) was also rounded or pear-shaped, but anchored itself by the tapering end of its body. The sea lilies (the red ones on extreme left) belong to the same group. Brachiopods (white shells, right) furnish the most abundant invertebrate remains in Ordovician sediments. Cephalopods (nautilus) were the early version of the present-day cuttlefish. They are active hunters with arms

Figure 02a Ordovician Period [view large image]

(tentacles) lie in front of the eyes and a parrot-like beaks. Their body is enclosed in shell. When they needs to move swiftly, the mantle cavity would be filled with water and squirts it out through a tube.

The biodiversity at the Cambrian Period actually flourished for about 20 million years, then reversed to a decline for some unknown reasons. The real bio-diversification happened during the Ordovician (see Figure 02b). Some attributes the bloom to favorable environment such as stable climate and plenty of shallow water etc. Other claims that it is the increasing asteroid impacts that created ecological niches for life to evolve into.

Figure 02b Biodiversity [view large image]

• At the end of the Ordovician period the climate turned cold, 85% of all species on Earth including many trilobites, brachiopods and nautiloids disappeared in what was the first major mass extinction (Figure 02b).

Go to Next Section
 or to Top of Page to Select
 or to Main Menu